GB543709A - Improvements in television transmission of pictures - Google Patents

Abstract

543,709. Television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 31, 1940, No. 13696. Convention date, Aug. 31, 1939. [Class 40 (iii) ] A television transmission system utilizes a cathode-ray pick-up tube in which the electrons of an electron beam scanned over a mosaic of electron-emissive capacity elements approach zero velocity at the point of impact on the mosaic as described in Specifications 542,219 and 530,409 ; the image of a picture-film or the like is projected on the mosaic momentarily during the return-trace period between successive scannings of the mosaic, thus producing integrated blacker than black impulses x, Fig. 3, at frame frequency ; the mosaic thus locally charged in correspondence with the image is then scanned in successive lines, producing the picture signal v, the A.C. axis of the combined signal x, y being at black-level; the impulses x are clipped off at black-level in one or two stages to produce pedestals ; and synchronizing pulses are inserted on these pedestals in the blacker than black direction. In the transmitting circuit shown in Fig. 1, the picture is projected from a film 12 through a rotating disc 36 on the mosaic 11. The projection system may be as described in Specification 431,207 and U.S.A. Specification 2,082,093, and may be maintained in synchronism with the television signal by means of a control circuit 37 as described in Specification 458,161. Synchronizing pulses are generated in a circuit 26, which may be as described in Specification 448,065 or in U.S.A. Specification 2,132,665. This synchronizing generator 26 controls line and frame scanning generators 31, 28, an amplifier 35 for supplying line and frame blanking impulses, and an amplifier 57 for supplying the line and frame synchronizing pulses to the signal to be transmitted. The signal produced by the charging and discharging of the mosaic 11 is passed by means of a resistance 30 and a condenser to an amplifier 45 followed by an amplifier 50 where the signal is clipped, blanked, and mixed with the synchronizing pulses before passing to the transmitting amplifier 55. The clipping may be performed in one or two stages of negatively biased valves. If the switch 56 is opened thereby disconnecting the blanking amplifier 35 from the signal amplifier 50, the synchronizing signals occur on pedestals at black-level. It is preferred, however, to close the switch and arrange for the circuit to produce pedestals at a level slightly blacker than black. Lost D.C. may be reinserted in any known way, for example, by devices such as described in Specifications 422,906 and 434,496.